Electronic Article Surveillance (“EAS”) systems are detection systems that allow the detection of markers or tags within a given detection region. EAS systems have many uses. Most often EAS systems are used as security systems to signal shoplifting of items from stores or signal removal of property from office buildings. EAS systems come in many different forms and make use of a number of different technologies.
Typical EAS systems include an electronic detection unit, markers and/or tags, and a detacher or deactivator. The detection unit includes transmitter and receiver antennas and is used to detect any active markers or tags brought within range of the detection unit. The antenna portions of the detection units can, for example, be bolted to floors as pedestals, buried under floors, mounted on walls, or hung from ceilings. The detection units are usually placed in high traffic areas, such as entrances and exits of stores or office buildings. The deactivators transmit signals used to detect and/or deactivate the tags.
The markers and/or tags have special characteristics and are specifically designed to be affixed to or embedded in merchandise or other objects sought to be protected. When an active marker passes through the detection unit, an alarm is sounded, a light is activated, and/or some other suitable control devices are set into operation indicating the removal of the marker from the proscribed detection region covered by the detection unit.
Most EAS systems operate using the same general principles. The detection unit includes one or more transmitters and receivers. The transmitter sends a signal at defined frequencies across the detection region. For example, in a retail store, placing the transmitter and receiver on opposite sides of a checkout aisle or an exit usually forms the detection region. When a marker enters the region, it creates a disturbance to the signal being sent by the transmitter. For example, the marker may alter the signal sent by the transmitter by using a simple semiconductor junction, a tuned circuit composed of an inductor and capacitor, soft magnetic strips or wires, or vibrating resonators. The marker may also alter the signal by repeating the signal for a period of time after the transmitter terminates the signal transmission. This disturbance caused by the marker is subsequently detected by the receiver through receipt of a signal having an expected frequency, the receipt of a signal at an expected time, or both. As an alternative to the basic design described above, the receiver and transmitter units, including their respective antennas, can be mounted in a single housing.
Magnetic materials or metal, such as metal shopping carts, placed in proximity to the EAS marker or the transmitter may interfere with the optimal performance of the EAS system. Further, some unscrupulous individuals utilize EAS marker shielding to shoplift merchandise without detection by EAS system. Metal can shield tagged merchandise from the EAS detection system by preventing an interrogation signal from reaching the tags or preventing a reply signal from reaching the EAS system. When a shielded marker passes through the detection unit, the EAS system is not able to detect the marker. As a result, shoplifters are able to remove articles from stores without activating an alarm.
Metal detection systems are used in conjunction with EAS systems to detect the presence of metal objects such as shielded bags or shopping carts. The metal detection systems may use transmitters and receivers in common with the EAS system. For metal detection, the transmitter sends a signal across the detection region at a predefined metal detection frequency. When a metal object enters the detection region, it creates a disturbance to the signal being sent by the transmitter. This disturbance caused by the metal object is subsequently detected by the receiver through receipt of a modified signal. Upon detection of the modified signal, an alarm is sounded, a light is activated, and/or some other suitable control devices are set into operation indicating the presence of metal in a detection region.
The metal detection systems are sensitive to interference signals introduced through power lines, including sudden spikes in electrical current or other fluctuations on the power lines. Conventional metal detection systems are inclined to producing false alarm signals when subjected to power line interference signals. What is needed is a system and method of detecting and canceling power line interference signals in metal detection systems to reduce occurrences of false alarm trigger signals.